Update headings guidance in Nature Typography section

[amyhupe]

What

Add guidance on writing headings and using semantic heading levels correctly.

Why

We started documenting this as part of the Nature Section heading component, but it applies more widely than that.

Done when

• [x] guidance drafted
• [x] reviewed by @foxintherain
• [x] reviewed by developer with a11y knowledge
• [x] published

Anything else

• Draft guidance

[amyhupe]

Opened a PR for this guidance https://github.com/springernature/ux-elements-docs/pull/52 - have asked someone to review.